1. Field of the Invention
This invention relates to a system and method for performance of a Built-In Test (BIT) for a photoconductive photodiode such as may be used in a semi-active laser (SAL) seeker.
2. Description of the Related Art
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. Photodiodes are similar to regular semiconductor diodes except they are packaged either for direct exposure to light or packaged with a window or optical fiber connected to allow light to reach the sensitive part of the device. Photodiodes may detect light in from the UV band of approximately 0.3 microns to the LWIR band of approximately 12 microns.
A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, the photon excites an electron, thereby creating a mobile electron a positive charged electron hole. If the absorption occurs in the junction's depletion region, or within one diffusion length of the depletion region, these carriers are swept away from the junction by the built-in field of the depletion region. Thus holes move toward the anode and electrons towards the cathode and a photocurrent is produced.
Reverse biasing the photodiode causes it to operate in the photoconductive mode. The reverse bias increases the width of the depletion region, which decreases the junction's capacitance resulting in faster response time. The reverse bias induces a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. The photocurrent is linearly proportional to the incident light. In the photoconductive mode, the photodiode converts light at its anode into photocurrent at its cathode.
One use of photoconductive photodiodes is within semi-active laser (SAL) seekers to detect pulsed laser electro-magnetic radiation (EMR) scattered from an intended target to provide signals indicative of target bearing to guide a projectile (e.g. laser guided artillery shell, guided missile or guided bomb) to the target. The SAL may include an optical system to capture and focus the scattered laser EMR onto a quad-detector. The optical system may convert the target bearing to an irradiance distribution or “spot” positioned on the detector. As the target bearing changes the position of the spot on the detector changes. The quad-detector includes four photoconductive photodiodes that sense whether and to what extent the “spot” is positioned in each quadrant and produce a photocurrent signal accordingly. When the spot is centered on the detector, the photodiodes produce photocurrent signals that are essentially equal.
The SAL seeker must be periodically tested to ensure that the quad-detector is working and working properly. This is commonly referred to as a Built-In Test (BIT). The current state-of-the art is to include an additional laser diode as part of the seeker that emits a beam that scatters off of the optical system. The detector looks for and processes the backscatter off of the optical system to test the photoconductive photodiodes. This approach requires an additional piece of hardware in the laser diode, which introduces another failure mode, requires that the processing algorithms be adapted for the backscattered light as opposed to light reflected off the target and provides only limited BIT capability.